The present invention relates to the control of braking for a vehicle equipped with a regenerative braking means, for example an electric braking means, and with an additional friction-type braking means, for example a hydraulic braking means, notably on an electrically powered vehicle.
On a vehicle equipped with at least one electric traction or propulsion motor it is possible, under certain conditions, to use the electric motor as a generator and thus obtain an electric braking means. Such use is advantageous because, being regenerative, it allows some of the energy to be recovered in order to recharge the batteries.
An apportioning system has been designed to apportion an overall braking command between an electric braking command intended for an electric braking means and an additional braking command intended for an additional braking means. This apportioning system is designed to bring about certain transitions to all-electric braking or purely friction braking.
For example, in the event of the detection of a malfunctioning of the electric actuator, the electric braking is deactivated.
In another example, when the vehicle is running at relatively low speeds, the braking is entirely hydraulic. Provision is therefore made for electric braking to be progressively deactivated according to the speed of the vehicle.
FIG. 1 illustrates this transition in a situation in which the driver is applying a constant braking command so that the speed decreases over time. The instant t1 corresponds to a speed of 14 km/h, the instant t2 corresponds to a vehicle speed of 7 km/h. The solid curve corresponds to the electric braking setpoint and the dashed curve corresponds to the hydraulic braking setpoint. These two setpoints are determined by an apportioning system as described hereinabove, notably as a function of an overall braking setpoint corresponding to the pressure of the driver on the pedal.
As may be noted from FIG. 1, these two thresholds at 14 and 7 km/h correspond to a transition to hydraulic braking as the vehicle gradually decelerates.
Now, the various means of braking a same vehicle may have different response times. For example, an electric braking means may have a response time of the order of 100 milliseconds, whereas the response time for a hydraulic braking means and the means may be rather of the order of 200 milliseconds.
It therefore sometimes happens that the switchover from all-electric braking to all-hydraulic braking is accompanied by a short variation in the braking torque actually applied. The user may thus have a sensation of brake release, associated with such a hole in the deceleration.
Referring back to FIG. 1, the curve in dot-dash line corresponds to the actual deceleration of the vehicle. As can be seen, for speeds slightly below 14 km/h, there is a brief drop in the actual braking, even though the user is still applying constant pressure to the brake pedal.
This hole in the deceleration may be disturbing to the user who may find himself braking harder to compensate. This phenomenon is especially disturbing to the user since it will not occur at other speeds, for example at 50 km/h.
There is therefore a need for a method and a system that would improve user perception.
It has been envisioned to spread the transition described hereinabove over a broader range of speeds, for example between 20 and 7 km/h rather than between 14 and 7 km/h, so that the switchover occurs more slowly. Nevertheless, this leads to a limitation on the regenerative braking and therefore on the possibility of recharging the batteries.